1. Field of the Invention
The invention relates generally to the preservation of contact lenses. More particularly, the present invention relates to a method and composition for sterilizing plastic hydrophilic or "soft" contact lenses.
2. Background of the Prior Art
Hydrophilic or partially hydrophilic plastic materials have been described for use in making so-called soft contact lenses. For example, U.S. Pat. No. 3,503,393 to Seiderman and U.S. Pat. No. 2,976,576 to Wichterle describes processes for producing three-dimensional hydrophilic polymers of polyhydroxyethylmethacrylate in aqueous reaction media having a sparingly cross-linked polymeric hydrogel structure and having the appearance of elastic, soft, transparent hydrogels. Other soft contact lenses include lenses made out of other optically suitable materials.
The main virtues of these lenses are their softness and optical suitability. The hydrophilic lenses are particularly useful in ophthalmology due to their remarkable ability to absorb water with a concomitant swelling to a soft mass of extremely good mechanical strength, complete transparency and the ability to retain shape and dimensions when equilibrated in a given fluid.
One of the problems connected with these soft contact lenses is the method of their sterilization and cleaning. The very property of the hydrophilic soft lenses which allows them to absorb up to 150 percent by weight of water also allows preservatives which might otherwise be used for cleaning and sterilization to be absorbed and even concentrated and later released when the soft contact lens is on the eye. The release may be much slower than the uptake, thereby allowing the preservative to build up in the lenses. This buildup eventually affects the physical characteristics of the lenses including dimension, color, etc. This can have the harmful result of damaging or staining the contact lens itself and/or harming the sensitive tissues of the conjunctivae or cornea.
Hard contact lenses do not absorb appreciable amounts of water (i.e. 0.1-0.4%) and thus the use of effective preservatives generally does not create a problem in the hard contact lens field. Sterilization of hydrophilic soft contact lenses is carried out by procedures described in U.S. Pat. Nos. 3,689,673 or 3,888,782 or more generally by boiling the lenses in normal saline. Furthermore, users of soft contact lenses are warned that under no circumstances should solutions designed for hard contact lenses be used, for the reason that the preservatives in such solutions will be absorbed and even concentrated by the soft lens and may seriously damage the soft lens and/or the eye of the user. In this connection, U.S. Pat. No. 3,689,673 further discloses that a number of commonly used antimicrobial agents are concentrated in the soft lens and suggests that these materials may cause corneal damage and that similar in vitro and in vivo tests have shown the undesirability of such antimicrobial agents when used with hydrophilic lenses.
It is also known that the relatively rapid oxidation of an ene-diol compound effects substantially a complete kill of microorganisms which are in contact with it. Ordinarily, the oxidation of an ene-diol compound occurs too slowly to be effective as a microbicide. However, some metal ions, notably copper ions, catalyze the reaction so that it takes place rapidly and thereby is effective as a microbicide, e.g. see U.S. Pat. No. 3,065,139.
Commercial utility of the rapid oxidation of an ene-diol compound to obtain the benefit of its microbicidal effect is difficult because the reaction takes place so quickly when the necessary ingredients are in contact with each other that commercially packaged products do not have useful shelf life. U.S. Pat. No. 3,681,492 deals with compositions in which the rate of oxidation of ene-diol is inhibited or retarded so that the microbicidal effect is sustained for a longer period. However, even the retarded compositions generally are not useful as commercial products because the reaction can only be retarded for a relatively short time, for example a matter of days, which is not long enough to provide a suitable shelf life for a commercial product. In addition, a highly retarded reaction does not produce as intense an antimicrobial effect as a mildly retarded reaction or one which is not retarded. Other problems with the foregoing ene-diol reaction are that the breakdown products of common ene-diols, e.g. ascorbic acid, form a brownish color in solution and the copper ions in solution react with each other to form a red copper metal precipitate. Both of these problems render the prior art formulation unfit for use in preserving hydrophilic contact lenses.